The present invention relates to a method and apparatus for tensioning wire rope while used as a load hoist line on a crane.
The most common use of a crane is to lift objects from ground level to an elevated position. When lifting from the ground, the total crane load is the sum of the weights of the object, the rigging between the hook block and object, the hook block, and the wire rope below the boom top. The weight summation divided by the parts of load hoist line equals the load hoist average line pull. The load hoist lead line pull, which is the actual tension in the load hoist line at the drum, is slightly higher than the average line pull, due to friction and other small inefficiencies in the sheaves. When an object is lifted, and the rigging is first pulled tight, the weight of the object increases the lead line pull, assuring the load hoist rope spools tightly on the drum as the object is lifted.
Maintaining the proper lead line tension when spooling long lengths of rope has been an ongoing problem. When wire rope is first placed on a drum (either at the factory when a crane is built, or in the field when new line is being installed), the factory or the field installation crew uses a “hold back” device to put tension on the wire rope as it is spooled onto the drum. This assures that the wire rope is tightly spaced on the drum, and when a load is put on the line later, the rope will not cut into the layers below it.
However, some cranes are used to hoist an object where the object ends up at a lower elevation after the lift than at the beginning of the lift. Some typical examples of this is where a crane lets an object down a shaft into a tunnel. Another example is where a piece of equipment needs to be repaired or replaced, and that piece of equipment is at an elevated position compared to where it needs to be moved to, such as is a wind generator assembly, commonly referred to as a nacelle, on a support tower. The nacelle may need to be removed and lowered because of a component failure or to change out the nacelle to a more powerful or more efficient unit. A crane that may be used to pick the nacelle up off its tower and let it down to the ground may be rigged with a 90 meter (295 ft.) main boom plus a 7 meter (23 ft.) extended upper boom point. The hook block may be rigged with six parts of load hoist line. The load hoist wire rope length needed in this situation is 700 meters (2300 ft.). Even if the crane is rigged with the minimum load hoist wire rope length of 700 meters, minimizing the rope spooled on the drum, and thus minimizing the layers of rope on the drum, a typical load hoist drum with 700 meters of wire rope may have six layers of rope.
Considering a hoisting operation where the object is being moved from a higher elevation to a lower elevation, first the hook block and rigging have to be raised while there is only a minimal load hoist lead line pull. If the hook block is raised to a high elevation, the drum has six layers of very loosely spooled rope on it. When the object is attached to the hook block rigging and lifted off of its support, the load hoist lead line pull increases greatly. Spooling problems have been reported in these types of lifts when the object is lowered to the ground. Gaps in the rope on the drum seem to occur near the flanges and at the cross-overs. Rope pulling down into lower layers has also been reported.
Larger diameter rope spools better as long as the last wrap of the layer can fit into the space between the drum flange and the rope already on the drum. The larger the diameter the rope, up to the pitch between the lagging grooves, the tighter the rope is packed on the drum and the less room that is available for gapping. The tight wrapping of the rope also reduces the likelihood of the layer above to cut in when lifting an object. The rope diameter however cannot be too large. If it is larger than the pitch between lagging grooves it will not be able to fit into the lagging properly. Also, the rope deforms (ovalizes) as is it is wrapped around the drum and this increases its effective width on the drum. This increased width may prevent the last wrap from fitting properly onto the drum next to the flange, which will cause the rope to rise up to the next layer early.
Loose rope (installed with low tension) causes spooling problems even on low layers. The loosely spooled layers of load hoist rope cannot support the increased lead line pull. The lead line will force itself down (cut down) through several layers of rope. In the worst case, the lead line is forced under the outer layers. The outer layers then foul the lead line rope and keep it from unspooling. The object is now stuck in the air.
A number of different solutions to the problem of loosely spooled rope have been proposed. If a much larger drum diameter with fewer layers of rope were used, there would be fewer chances for the line to cut into the layers below it. However, this approach may not be practical, especially for large cranes that are designed to be partly disassembled for transportation over the highway between job sites, as those cranes are typically already designed for maximal highway limits. Additionally, a larger drum is more expensive, and increases the size of other components on the crane, making the crane harder to maneuver on the job site.
Other suggestions include efforts to put frictional forces either on the rope itself, or on pulleys that then engage the rope, to increase the rope tension when the hook block is being raised without an object attached to it. Ideas in this category include a traction winch on the crane (rope wraps multiple times around two wheels), and brake blocks or wheels squeezing the rope. Each of these concepts has drawbacks. Engaging a frictional force against the rope adds to the wear on the rope, which in turn reduces the useful life of the rope. Systems that wrap the rope around additional pulleys create more bending in the rope, once again reducing the useful life of the rope, especially where the diameter of the pulleys are small.
Thus it would be a great advantage if a rope tensioning system could be developed that allows a crane on a job site that needs to perform a lift where an object has to be lowered, particularly when using a long rope length, to somehow get the load hoist line tight on the drum before the object is lowered, without adding extra bending motions in the travel path of the rope or engaging the rope with frictional forces.